1. Field of the Invention
The present invention relates to a rotator attachment for a lift truck in which rotation occurs through the use of multiple hydraulic cylinders to drive a common gear member in a rotary path, and more particularly to such a rotator having pressure-relieved valving for operating the cylinders in correct sequence and for preventing pressure-locking in the cylinders.
2. Description of the Prior Art
The closest known prior art is U.S. Pat. No. 3,876,100, disclosing a rotator for lift truck attachments, of which the present invention is an improvement.
The rotator of such prior patent employs inner and outer gears in surrounding relationship and with the outer gear having internal teeth engaging external teeth of the inner gear, but with the outer gear having a greater number of teeth than the inner gear. One of the two gears is mounted for rotation about an axis. The other gear is driven in a gyratory eccentric path with respect to the one gear so as to rotate the one gear and a connected load-handling attachment.
According to one embodiment of the prior patent, the drive means for achieving the desired gyratory eccentric movement of one of the gears is a series of angularly disposed, double-acting hydraulic cylinders connected at one set of ends to a relatively stationary frame and at the other set of ends to the gear to be driven. The hydraulic cylinders are driven in correct sequence to effectuate the desired gyratory eccentric movement of the gear. Sequencing of the cylinders occurs through use of spool valves connected to the cylinders and shifting in response to cylinder oscillation. The spool valves are basically three-position valves, including a center neutral position between two operating positions. The problem with this arrangement is that when one driving cylinder is in or near its maximum power position another driving cylinder is in or near a bottom or top dead-center position, changing from a retraction to an extension mode, or vice versa. In such transitional positions of the inactive cylinder, its associated control valve is passing through its flow-blocking position. Yet, the inactive cylinder is still being driven by the powered cylinder. Therefore, excessive fluid pressure builds up on one side of the inactive cylinder as it tries to retract or extend with its control valve in a neutral position. This so-called pressure-locking in the inactive cylinder can break its piston rod unless the excessive pressure is relieved. To avoid pressure-locking in the drive cylinders of the prior patent, a bleed orifice is provided in the valve spool of the control valve for each cylinder. This orifice allows enough flow between the opposite side of each cylinder to prevent excessive pressure buildup in the cylinder. Such orifices are shown at 120 and 121 in the valve spools 82, 88 in FIG. 5 of the aforementioned patent. While the use of such a constant bleed path between opposite sides of each cylinder in theory should be an adequate solution to the pressure-locking problem, in practice it has been found to be unsatisfactory because of the difficulty in maintaining an adequate working pressure in the hydraulic system.
Accordingly, there is a need in hydraulic cylinder-driven rotators of the type described to provide an improved means for preventing pressure locking of the cylinders when they change modes.
Also, in rotators of the described inner-outer gear type, as disclosed in the aforesaid prior patent, there is a need to reduce the bulk and weight of the rotator so as to increase the maximum pay load of the lift truck. For example, in the hydraulic cylinder-driven version of the rotator shown in FIG. 2 of the aforementioned patent, four hydraulic cylinders, each with a separate control valve assembly, and four eccentric mountings are used to drive the outer gear plate of the rotator. This produces excessive bulk and weight. The prior patent suggests that as few as two double-acting hydraulic cylinders would be sufficient to drive one of the gears of the rotator in a gyratory eccentric path. Although this is true, how it could be accomplished is not disclosed. Accordingly, there is a need to provide a rotator of the inner-outer gear type described with minimum weight and bulk for optimum payload.